1. Field of the Invention
This invention relates to the chemical analysis of substances, and more particularly, to apparatus for the precise metering of biological fluids onto test elements.
2. State of the Prior Art
A number of automated systems have been developed for performing quantitative chemical analyses of fluid samples. Most of the commercially available systems utilize liquid reagents and require analyzer equipment having intricate solution handling and transport capabilities. Recent developments, however, have provided test elements in essentially planar, dry form which can be loaded into a cartridge for use in an analyzer. In the use of such an analyzer, a test element from a cartridge is fed into a metering station where a predetermined amount of sample fluid is deposited on the test element. After an incubation period, the element is moved to a read station where a change in the test element is measured, the amount of change being proportional to a particular analyte in the fluid. The test element is used only once and is discarded after the reading has been taken. An analyzer for use with such test elements is disclosed in commonly-assigned U.S. Pat. No. 4,152,390, granted May 1, 1979.
Test elements of the type described above are adapted to function with very small quantities of fluid. For example, test elements for performing colorimetric analyses can produce a measurable response with only 10 microliters of sample fluid, and elements for performing potentiometric analyses are operable with 10 microliters of sample fluid and 10 microliters of reference fluid. Very precise metering devices are required for use with the test elements, since the volume of fluid supplied to the elements should preferably not vary more than five percent from a selected value to achieve desirable test results. Further, in automated and in multi-channel analyzers the metering devices must be capable of repeatedly and accurately dispensing micro-quantities of fluid onto a series of test elements.
A metering device for use with planar test elements is shown in commonly-owned U.S. Pat. No. 4,142,656, granted to Smith et al., on Mar. 6, 1979. In this patent, fluid is dispensed from a sample cup having a dispensing tip formed on a bottom wall thereof. An electrically-actuated pump is used to generate a pressure in the cup sufficiently above ambient to form a pendant drop on the cup. The test element is then moved into contact with the pendant drop to effect a transfer of the fluid to the element. Both the sample cup and the test element are transported to the metering apparatus. The metering device disclosed in the Smith et al. patent requires complex transport and drive elements for both the sample cup and the test element, and it is not intended for use in applications where fluid must be aspirated into the metering system.
There is a need for precise metering apparatus which is suitable for use on relatively small, portable analyzers. Such analyzers are used in operating rooms for emergency tests, or carried in ambulances for field use. Preferably, the metering apparatus for these analyzers should be simple and manually actuated, since power may not be available. Known metering apparatus of this type often lacks the precision required for acceptable results. One of the main problems with manually-actuated metering devices is that the volume of fluid delivered varies with the speed of actuation; thus, results vary with different operators and from one metering operation to the next. Patents relating to relatively simple metering devices include: U.S. Pat. No. 3,683,977, granted on Aug. 15, 1972; and U.S. Pat. No. 4,110,167, granted on Aug. 29, 1978. cl OBJECTS OF THE INVENTION
It is an object of the present invention to overcome the above-described problem in prior-art devices, and to provide a novel and improved apparatus for the repeated, precise dispensing of micro-quantities of fluid.
It is another object of the invention to provide apparatus for the precise dispensing of a predetermined quantity of fluid in which the apparatus is adapted to expel a portion of said quantity in a first period of a dispensing cycle and to expel the remainder of said quantity in a second period of the cycle.
Yet another object of the invention to provide apparatus for the precise dispensing of a predetermined quantity of fluid from a metering tip in which the apparatus is adapted to create a first pressure condition in the metering tip in a first period of a dispensing cycle and to create a second pressure condition in the tip in a second period of the cycle to expel the fluid from the tip.
Still another object of the invention is to provide means in a metering apparatus for discharging a quantity of fluid immediately after aspiration to insure that the apparatus is primed for a precise metering operation.
A further object of the invention is to provide a manually-actuated metering apparatus which has a dispensing cycle of uniform duration.
A still further object of the invention is to provide a metering apparatus which is particularly suitable for substantially simultaneously metering predetermined quantities of a sample fluid and a reference fluid onto a substrate.
Other objects and advantages will become apparent from the following Summary and Description of the Preferred Embodiments, when considered in the light of the attached drawings.